The proposed research in experimental animals is designed to 1) determine the relationship of splenic mass to immune function of the spleen, and 2) develop and evaluate therapeutic alternatives to splenectomy which reduce excessive or normal splenic function. Indices of immune function (hemolysin titers, IgG and IgM hemolytic plaque-forming cells, and clearance of pneumococcal bacteria in the presence and absence of opsonins) will be examined in rats after removal, reduction (autotransplantation of graded weights of splenic tissue, splenic artery ligation or splenic artery embolization), and augmentation (methyl-cellulose-induced splenomegaly) of splenic mass in comparison to normal animals. In this way it will be determined whether the immune response in these animals correlates directly with the mass of intact spleen and whether there is a critical mass below which the spleen no longer provides detectable immune protection. This project should also shed light on the relationship between blood flow and immune function in the spleen as well as on the changes in splenic function which occur in a variety of disease states. At the same time these data may provide a further experimental basis for "circulatory control" of hypersplenism in benign and malignant disorders and for control of tumor growth in organs other than the spleen.